The problem that killed SOAP, to my mind, was growing complexity that eventually started to outweigh the benefits of the protocol. Not every implementation supported every new complication that, say, MS kept inventing, it hurt interoperability.
I think a modern sort-of-replacement for the SOAP's use case is GRPC. It eschews the heavyweight XML and tries to keep things simple, while staying reasonably typesafe, declarative, discoverable, and extendable.
What would you suggest, beyond the standard "ReST" and GraphQL, with OpenAPI as a declarative description?
I think the ser/des is slower than JSON in most browsers, but the message format is smaller.
Oftentimes, using a query parameter like `?exclude[]=…` or `?include[]=…` or similar to say "only get me these response fields, not the whole object" can be useful for this too (and then you still get JSON back).